Transforming growth factor alpha in arterioles: cell surface processing of its precursor by elastases

Inhibition of epidermal growth factor receptor attenuates atherosclerosis via decreasing inflammation and oxidative stress

Atherosclerosis is a progressive disease leading to loss of vascular homeostasis and entails fibrosis, macrophage foam cell formation, and smooth muscle cell proliferation. Recent studies have reported that epidermal growth factor receptor (EGFR) is involved vascular pathophysiology and in the regulation of oxidative stress in macrophages. Although, oxidative stress and inflammation play a critical role in the development of atherosclerosis, the underlying mechanisms are complex and not completely understood. In the present study, we have elucidated the role of EGFR in high-fat diet-induced atherosclerosis in apolipoprotein E null mice. We show increased EGFR phosphorylation and activity in atherosclerotic lesion development. EGFR inhibition prevented oxidative stress, macrophage infiltration, induction of pro-inflammatory cytokines, and SMC proliferation within the lesions. We further show that EGFR is activated through toll-like receptor 4. Disruption of toll-like receptor 4 or the EGFR pathway led to reduced inflammatory activity and foam cell formation. These studies provide evidence that EGFR plays a key role on the pathogenesis of atherosclerosis, and suggests that EGFR may be a potential therapeutic target in the prevention of atherosclerosis development.

Converting enzyme-independent release of tumor necrosis factor alpha and IL-1beta from a stimulated human monocytic cell line in the presence of activated neutrophils or purified proteinase 3

Two important cytokines mediating inflammation are tumor necrosis factor alpha (TNFalpha) and IL-1beta, both of which require conversion to soluble forms by converting enzymes. The importance of TNFalpha-converting enzyme and IL-1beta-converting enzyme in the production of circulating TNFalpha and IL-1beta in response to systemic challenges has been demonstrated by the use of specific converting enzyme inhibitors. Many inflammatory responses, however, are not systemic but instead are localized. In these situations release and/or activation of cytokines may be different from that seen in response to a systemic stimulus, particularly because associations of various cell populations in these foci allows for the exposure of procytokines to the proteolytic enzymes produced by activated neutrophils, neutrophil elastase (NE), proteinase 3 (PR3), and cathepsin G (Cat G). To investigate the possibility of alternative processing of TNFalpha and/or IL-1beta by neutrophil-derived proteinases, immunoreactive TNFalpha and IL-1beta release from lipopolysaccharide-stimulated THP-1 cells was measured in the presence of activated human neutrophils. Under these conditions, TNFalpha and IL-1beta release was augmented 2- to 5-fold. In the presence of a specific inhibitor of NE and PR3, enhanced release of both cytokines was largely abolished; however, in the presence of a NE and Cat G selective inhibitor, secretory leucocyte proteinase inhibitor, reduction of the enhanced release was minimal. This finding suggested that the augmented release was attributable to PR3 but not NE nor Cat G. Use of purified enzymes confirmed this conclusion. These results indicate that there may be alternative pathways for the production of these two proinflammatory cytokines, particularly in the context of local inflammatory processes.

The carboxyl-terminal valine residues of proTGF alpha are required for its efficient maturation and intracellular routing

Soluble forms of transforming growth factor-alpha (TGF alpha) are derived by proteolytic processing of an integral membrane glycoprotein precursor (pro TGF alpha). Previous studies indicated that phorbol ester-induced cleavage of pro TGF alpha in CHO cells is dependent on the presence of a valine residue located at the carboxyl terminus of the precursor's cytoplasmic domain. We reassessed this requirement with epitope-tagged constructs introduced into transformed rat liver epithelial cells that normally express and process TGF alpha. We found that pro TGF alpha mutants lacking the terminal valine residues showed greatly reduced maturation to the fully glycosylated form. Additionally, they were present at substantially reduced levels on the cell surface and, instead, accumulated in the endoplasmic reticulum. Consistent with these results, enzyme-linked immunosorbent assay (ELISA) and Western blot analyses revealed little or no soluble TGF alpha in medium conditioned by cells expressing the mutant constructs. Finally, a truncated pro TGF alpha mutant lacking most of the cytoplasmic domain but retaining a carboxyl-terminal valine was processed and cleaved in a near-normal manner. These results, some of which were reproduced in CHO cells, indicate that the predominant effect of the carboxyl-terminal valines is to ensure normal maturation and routing of the precursor.

Elafin, a serine elastase inhibitor, attenuates post-cardiac transplant coronary arteriopathy and reduces myocardial necrosis in rabbits afer heterotopic cardiac transplantation

We have related experimentally induced post-cardiac transplant coronary arteriopathy to increased elastolytic activity, IL-1beta, fibronectin-mediated inflammatory and smooth muscle cell (SMC) migration, and SMC proliferation. Since our in vitro studies show that a serine elastase releases SMC mitogens and facilitates IL-lbeta induction of fibronectin, we hypothesized that administration in vivo of the specific serine elastase inhibitor, elafin, would decrease the post-cardiac transplant coronary arteriopathy. Cholesterol-fed rabbits underwent a heterotopic cardiac transplant without immunosuppression and received elafin (1.79 mg/kg per d continuous infusion after a 9 mg bolus, n = 6) or vehicle (n = 6). 1 wk later, hearts were harvested for morphometric, immunohistochemical, and biochemical analyses. A > 70% decrease in the total number of coronary arteries with intimal thickening in elafin-treated compared to control donor hearts (P < 0.002) was associated with reduced vascular elastolytic activity judged by fewer breaks in the internal elastic lamina (P < 0.03), less accumulation of immunoreactive fibronectin (P < 0.02), and reduced cell proliferation quantified by proliferating cell nuclear antigen (P < 0.0001). Despite myocardial lymphocytic infiltration, wet weight of elafin-treated donor hearts was reduced by 50% compared to untreated controls (P < 0.002) and associated with relative preservation of myocyte integrity, instead of extensive myocardial necrosis (P < 0.004). This protective effect correlated with decreased myocardial elastolytic activity (P < 0.0001) and inflammatory cell proliferation (P < 0.0001) and with an elafin-inhibitable elastase in lymphocytes. Serine elastase activity thus appears an important therapeutic target for post-cardiac transplant coronary arteriopathy and myocardial necrosis induced by rejection.

Three human elastase-like genes coordinately expressed in the myelomonocyte lineage are organized as a single genetic locus on 19pter

The human neutrophil and monocyte-derived serine protease homologues neutrophil elastase (NE), proteinase 3 (PR3), and azurocidin (AZU) are involved in a variety of immune defense reactions. NE and PR3 assist in the destruction of phagocytosed microorganisms, cleave the important connective-tissue protein elastin, and generate chemotactic activities by forming alpha 1-proteinase inhibitor complexes and elastin peptides. AZU is cytotoxic to certain microorganisms and chemotactic for monocytes. All three proteins are produced and packaged into azurophil granules in large quantities during neutrophil differentiation. We have isolated several cosmid clones each of which contains the functional genes for AZU, PR3, and NE in this order. The PR3 gene is separated by 8 kilobases from the 3' end of the AZU gene and by 3 kilobases from the 5' end of the NE gene. We report a physical map of the gene cluster, its location on chromosome 19pter, and the exon-intron organization of the AZU and PR3 genes. Our fluorescence in situ hybridization studies disprove the previous chromosomal assignment of the human NE gene to 11q14. The five exons of AZU and PR3 are organized like those of NE and other granule-associated serine proteases of hematopoietic cells. NE, PR3, and AZU are coordinately downregulated in the premonocytic cell line U937 during induced terminal differentiation. The cluster-like physical organization of these genes and concerted regulation during hematopoietic differentiation suggests that they are located in a developmentally activated chromatin domain promoting high-level, cell-specific expression in the monocyte-myelocyte lineage.

Glucose and glucosamine regulate growth factor gene expression in vascular smooth muscle cells

We have investigated the regulation of the expression of two growth factors found in vascular smooth muscle, transforming growth factor alpha (TGF alpha) and basic fibroblast growth factor (bFGF). Cells cultured in medium containing 30 mM glucose exhibited a 2-fold increase in TGF alpha mRNA and a 3-fold increase in bFGF mRNA compared with cells grown in normal (5.5 mM) glucose. Glucosamine was more potent than glucose, leading to a 6-fold increase in TGF alpha mRNA. TGF alpha protein levels were also increased by glucosamine treatment, and the predominant species present was the membrane-bound precursor form of TGF alpha. To examine further the regulation of growth factors by sugars, cultured rat aortic smooth muscle cells were transfected with a plasmid construct consisting of a 1.2-kilobase-pair fragment of the TGF alpha promoter linked to a luciferase reporter gene. Increasing the concentration of glucose in the culture medium from 5.5 mM to 30 mM led to a rapid, 1.7-fold increase in the activity of the TGF alpha promoter. Glucosamine was much more potent than glucose in this stimulation, with 2 mM glucosamine causing a 12-fold increase in TGF alpha promoter activity. Insulin had no effect on luciferase activity in either the presence or the absence of added sugars. The glucose response element of the TGF alpha gene maps to a 130-base-pair segment that includes three potential binding sites for the transcription factor Sp1. We conclude that high glucose concentrations such as are reached in diabetes mellitus can stimulate the transcription of the genes for growth factors in vascular smooth muscle cells. This signaling pathway apparently involves the metabolism of glucose to glucosamine. This effect could be representative of nutritional regulation of a family of genes and could contribute to the toxicity of hyperglycemia and the vascular complications of diabetes.